Question

An environmental chemist working for the Environmental Protection Agency (EPA) was directed to collect razor clams from a heavily-contaminated river superfund site and analyze them for cadmium (Cd2 ) content using graphite furnace atomic absorption spectrometry (GFAAS). The chemist dried the clams at 95° C overnight and ground them in a scientific blender, resulting in ~ 50 g of homogenized dry weight. A representative 91.65 mg sample was taken from the ~ 50 g of dry material and dissolved in 100.00 mL of 0.1 M HCl to create a sample solution. Using the method of standard additions, the chemist prepared five standard solutions in 100.00-mL volumetric flasks, each containing 5.00-mL aliquots of the sample solution. Varying amounts of a 96.0 ppb (?g ·L-1) cadmium standard were added to each of the flasks, which were then brought to volume with 0.1 M HCl. The solutions were then examined for their Cd2 content using GFAAS, resulting in the following absorbance data.

Use this table/data"

             Sample vol mL                                  Cd2+ Standard vol (mL)                                 Absorbance

                5                                                         0                                                            0.080

                5                                                       2.50                                                        0.119

                5                                                       5.00                                                        0.163

                5                                                       7.50                                                        0.200

                5                                                     10.00                                                        0.241

Determine the amount of Cd2 per gram of dry clam. Express your final result as milligrams of Cd2 per gram of dry clam.

------------------mg Cd2+/g clam

Answer 1

Ans. Part A: Determining [Cd²⁺] in 100.0 mL aliquot using standard addition method.

The trendline equation for the Abs vs concertation graph is “y = 0.0168x + 0.08” in the form of y = m x + c. where, y-axis = Absorbance, and x-axis = [Cd²⁺] in ppb; Y-Intercept = c ; Slope = m

Now, Concertation of un-spiked aliquot = (Intercept / Slope) concertation units

= (0.08 / 0.0168) ppb

= 4.762 ppb

Hence, [Cd²⁺] in 100.0 mL un-spiked aliquot = 4.762 ppb

Part B: The sample preparation (from fresh clams to final aliquot analyzed GFAAS) includes following steps-

Step 1: Fresh clams dried to a mass of 50.0 g (approx..)

Step 2: 91.65 mg of above dried sample is diluted upto 100.0 mL. Label it as solution 1.

Step 3: 5.0 mL of solution 1 is mixed with standard Cd²⁺ according and final volume made upto 100.0 mL. Label it as solution 2. Solution 2 is directly analyzed in GFAAS.

# Calculating total Cd²⁺ content in solution 2

Total amount of Cd in un-spiked solution 2 = [Cd²⁺] x Volume in liters

                                                                        = (4.762 ug/ L) x 0.100 L     ; [1 ppb = 1 ug/L]

                                                                        = 0.4762 ug

# Calculating [Cd²⁺] and Total Cd²⁺ content in in solution 1: 100 mL of solution 2 is prepared from 5 mL of solution 1. So, total Cd content of un-spiked solution 2 is equal to the amount of Cd in 5 mL of solution 1.

That is, Cd content of solution 1 = 0.4762 ug / 5 mL = 0.09524 ug/ mL

Now,

Total amount of Cd in solution 1 = [Cd²⁺] of solution 1 x volume of solution 1

                                                            = (0.09524 ug / mL) x 100 mL

                                                            = 9.524 ug

# Calculating [Cd²⁺] content in dry sample: Sample 1 is prepared from 91.65 mg dried clam. So, total Cd content of solution 1 is equal to total Cd content in 91..65 mg clam sample.

That is, Cd content of dried clam = 9.524 ug Cd²⁺ / 91.65 mg clam               

                                                            = (9.524 x 10^-3) mg Cd²⁺ / (91.65 x 10^-3) g clam

                                                            = 0.104 mg Cd²⁺ / g clam

Therefore, Cd²⁺ content in clam = 0.104 mg Cd²⁺ / g clam    (by dry mass)